1. Field of the Invention
The present invention relates to a piezoelectric driven MEMS device.
2. Related Art
Recently, variable capacitors and switches using actuators fabricated by using the MEMS (Micro-electro-mechanical System) technique are drawing the attention. A movable electrode is formed on a beam which is supported in the air over a substrate. A fixed electrode is formed on an opposed substrate. The beam is driven by electrostatic force, electro-thermal force, electromagnetic force, piezoelectric force, and so on. The distance between the movable electrode and the fixed electrode is changed. A variable capacitor or a switch is thus formed.
Especially in a variable capacitor or a switch which uses an inverse piezoelectric effect as drive force for the movable beam, the spacing between the movable electrode and the fixed electrode can be changed sharply and continuously, and consequently the capacitance change rate can be made large. Furthermore, since air or gas is used as a dielectric, a very large Q factor is obtained. Thus, there are a large number of advantages.
Furthermore, it is also possible to cause the variable capacitor structure to function as a switch by using it as it is and bringing the movable electrode into contact with the fixed electrode via an extremely thin dielectric film (capacitive type) or bringing the movable electrode into contact with the fixed electrode directly (DC type). Such a switch fabricated by using the MEMS technique has both low on-resistance and high off-time insulation isolation characteristics as compared with the semiconductor switch, and consequently it is also drawing keen attention.
However, the MEMS device using the piezoelectric drive mechanism is supported in the air, and has a long thin beam structure including a piezoelectric film interposed between upper and lower electrodes. Therefore, there is a very serious problem that the beam is warped upward or downward by slight residual stress in the material of the beam. As a result, it is very difficult to adjust the capacitance value of the variable capacitance obtained before and after applying the voltage in conformity with the design and keep the drive voltage of the switch at a constant value.
A magnitude of a movable displacement at an action end of a piezoelectric driven actuator is approximated by the following expression.Displacement caused by electrostrictive effect˜E·d31·L2·t−1  (1)Here, E is an electric field applied to the piezoelectric film, d31 is an electrostrictive constant, L is the length of the actuator, and t is the thickness of the actuator.
On the other hand, denoting residual strain by εr, warp of the piezoelectric driven actuator caused by residual strain which is generated in the piezoelectric film is approximated by the following expression.Warp caused by residual strain˜εr·L2·t−1  (2)
As appreciated by comparing the expression (2) with the expression (1), they have similar relations. The displacement and the warp are in proportion to the square of the length of the actuator, and are inverse proportion to the thickness. In other words, if the length of the actuator is increased or the thickness is decreased in order to widen the drive range of the piezoelectric driven actuator, the quantity of the warp also increases accordingly. In making the piezoelectric driven range greater than the warp, therefore, geometric contrivance concerning the actuator brings about little effect. There are no ways other than making the absolute value of the residual strain εr small as compared with the absolute value of strain (E·d31) caused by the electrostrictive effect.
For obtaining a fine film quality as regards PZT (lead zirconate titanate) known as a piezoelectric film material having a great electrostrictive effect, it is necessary to form a film at the room temperature and then conduct annealing at approximately 600° C. Since the volume contraction is caused by the annealing, the residual strain increases inevitably.
On the other hand, AIN (aluminum nitride) and ZnO (zinc oxide) used as the material of the piezoelectric film, which can be formed near at the room temperature and can be controlled in residual strain comparatively precisely by using the film forming condition at the time of film forming, are smaller in electrostrictive effect by at least one order than PZT.
Therefore, there are conflicting problems: a material that is great in electrostrictive effect is difficult to control the residual strain and a material that is comparatively easy in control of residual strain is small in electrostrictive effect. This is one of the greatest problems that obstruct industrial application of the piezoelectric driven MEMS device.
The present inventors have already devised a piezoelectric driven actuator having a folded structure in order to cope with this problem (see JP-A 2006-87231 (KOKAI)). The piezoelectric driven actuator having the folded structure includes a first beam which has a first end fixed to a substrate, a second end serving as a connection end, and a piezoelectric film interposed between a pair of electrode films, a second beam which has a first end serving as a connection end and a second end serving as an action end, which extends in a direction opposite to that of the first beam, and which has basically the same structure and dimensions as those of the first beam, and a fixed electrode disposed on the substrate so as to be opposed to the action end. In other words, two beams having the same structure and shape are disposed in parallel and ends of the two beams are connected together to form the folded structure. Even if warps are caused in the beams by residual strain at the time of film forming, therefore, the two beams warp in the same way. As a result, it becomes possible to cancel the warps. Thus, it has been anticipated that the distance between the action end of the piezoelectric driven actuator and the fixed end fixed to the substrate is kept substantially constant and consequently stable operation is made possible.
As a result of subsequent repetition of trial production and study conducted by the present inventors, it has become clear that it is possible in the simple folded structure or a folded structure having two forward beams on both sides and one backward beam in the center to reduce the warp caused at the action end by residual stress to approximately one tenth of the warp at the folding end, but it is still difficult to further hold down the warp at the action end. In short, the beams warp in a paraboloidal form around the fixed end. Although warps in the length direction of the actuator (extension direction of the beams) are canceled by the folded structure, therefore, warps of the actuator in the lateral direction are present and the warps are not canceled.